As is known, the lead frame is formed of a plurality of leads extending between an inner lead end for connection to the semiconductor die and an outer lead end for connection to a pin. The inner lead ends are relatively closely spaced together and arranged around the perimeter of a "window", in which the semiconductor die is to be bonded and the outer lead ends are relatively more separated than the inner lead ends so as to facilitate connecting the outer lead ends to individual pins of a semiconductor package.
Although the lead frame is mounted on a tape substrate for ease of handling on an automated manufacturing line, the lead frames are nevertheless somewhat fragile and can easily be damaged so as to have bent or broken leads or foreign matter inserted between the leads. Usually, the die and lead frame are tested after bonding and any such damaged devices can be rejected. But, if the rejection only occurs after the bonding step, then the whole device, including the die, is rejected, even if the fault lies in the lead frame.
Accordingly, it has been proposed to inspect the tips of the inner lead ends prior to bonding. An Inner Lead Bonder (ILB) manufactured by Shinkawa includes a visual inspection system for pre-bonding lead defect detection. The system can be programmed either to inspect all leads (100% inspection) or perform random checking at positions that are chosen manually. The followings steps are used to program the system to operate random checking:
1. Locate a first fiducial mark, i.e. the inner lead frame corner. PA1 2. Locate a second fiducial mark, i.e. the inner lead frame corner at the other side. By using two fiducial marks, the system can handle both translation and rotation of the inner frame under inspection. PA1 3. Learn the first lead group at the chosen region. The lead tip centers are computed at this stage. PA1 4. Repeat step 3 until all required areas are covered. PA1 a tape guide for receiving and guiding a tape substrate having lead frames mounted at intervals therealong; PA1 a first camera for imaging a first portion of a lead frame when the tape substrate is at a first position in the tape guide; PA1 a second camera for imaging a second portion of the lead frame when the tape substrate is at a second position in the tape guide; PA1 at least one lighting unit for lighting the tape substrate at the first and second positions from behind the tape substrate relative to the first and second cameras; PA1 an inspection computer coupled to the first camera to receive a first image of the first portion of the lead frame and coupled to the second camera to receive a second image of the second portion of the lead frame, the inspection computer including an inspection unit for detecting the leads in the first and second images and for generating an alarm signal if it detects broken or bent leads or foreign matter bridging between leads. PA1 a source of tape substrate having lead frames mounted at intervals therealong, the lead frames incorporating leads; PA1 at least one tape guide for receiving and guiding the tape substrate from the source to position adjacent lead frames at first and second stations; PA1 a first camera at the first station for taking a first image of a first portion of a lead frame when the lead frame is at the first station; PA1 a second camera at the second station for taking a second image of a second portion of the lead frame when the lead frame is at the second station; PA1 an inspection computer coupled to the first camera to receive the first image and coupled to the second camera to receive the second image, the inspection computer including an inspection unit for detecting the leads in the first and second images and for generating an alarm signal if it detects broken or bent leads or foreign matter bridging between leads; PA1 a bonding unit at a third station for receiving the lead frames on the tape substrate from the inspection apparatus and for automatically bonding semiconductor dies to the received lead frames, the bonding unit being controlled to stop operation if the inspection computer has generated the alarm signal. PA1 guiding a first lead frame on the tape substrate to a first station having a first camera thereat; PA1 taking a first image of a first portion of the first lead frame using the first camera and transmitting the first image to an inspection computer; PA1 guiding the first lead frame to a second station having a second camera thereat; PA1 taking a second image of a second portion of the first lead frame using the second camera and transmitting the second image to the inspection computer; PA1 inspecting the inner ends of the leads in the first and second images by the inspection computer and generating an alarm signal if broken or bent leads or foreign matter bridging between leads is detected; PA1 guiding the first lead frame to a third station having a bonding unit thereat; and PA1 bonding a semiconductor die to the first lead frame if no alarm signal has been generated by the inspection computer. PA1 overlaying scanlines perpendicular to the inner ends of the leads on each of the first and second images; PA1 detecting whether each of the leads extends over all the required scanlines and generating the alarm signal if not all the leads extend over all the required scanlines; PA1 overlaying scanlines parallel to the leads on each of the first and second images; and PA1 detecting whether any of the scanlines are crossed by the leads or other matter and generating the alarm signal if one or more of the scanlines are crossed.
For 100% inspection, the system will go via all the leads by using the above learning procedures. During operation, the two fiducial marks are first located so that the translation and rotation parameters can be obtained. Then a camera will move to each pre-programmed region using the offset parameters obtained previously. The leads at each inspection region will be found from each local window learned previously. If there is no lead found in the window, it prompts a broken lead defect. If all the leads are found in this region, it then computes the lead spacing between leads in order to detect lead bent defect.
However, this Shinkawa system uses a single camera to image the lead frame from above and inspects the lead frame part-by-part by moving the camera, thus requiring substantial time for a full inspection of each lead frame. Since the visual inspection process is embedded in the whole manufacturing process, utilising the vision system reduces the throughput of the bonding machine by about 40%. Although the Shinkawa system can inspect any part of the inner lead ends, it can only inspect a small region at a time and therefore requires several inspection steps to cover the entire inner lead ends. Furthermore, since the Shinkawa system is designed to inspect only the inner lead ends, defects such as foreign material inserted near the Polyimide Window are ignored. This results in undetected defects in lead frames, which are thus utilised in the die bonding process and incurs unnecessary waste of die.